Optical design for AO relay & coronagraph

1. Optical design for AO relay & coronagraph

2. Overview/status • Requirements list • Will be distributed (too long to go into great detail here) • Key points summarized next • AO relay • (likely) workable optical design in-hand • efforts underway to reduce number and complexity of optics • Coronagraph • (likely) workable optical design in-hand • Already at minimum optics count and complexity • To do: • Resolving lingering requirements issues • Fabrication issues • Tolerancing/optomechanics/alignment/operations plan

3. General requirements • 0.7-0.9 WFS band • 1.0-2.4 aggregate science band (no L-band) • 5 x 5 arcsec field • Throughput/background: very important—we currently have only placeholder values for total throughput and for non-uniformity across the pupil • We need to design with sufficient throughput margin so that we meet performance specs even as coatings degrade or optics collect dust over reasonable intervals • Big impact on mechanical design and operations (could require sealing, over-pressuring, frequent cleaning, or other measures to achieve performance specs); could have big impact on optical design • For reference, Gemini estimates ~0.17-0.35% loss/month for primary due to dust accumulation and up to 0-0.5% loss/month due to coating degradation (various types of protected silver)

4. AO relay requirements • 2 DM’s conjugate to telescope pupil • ~25 mm pupil, assuming 400 actuator spacing • Placing a DM at an altitude other than ground may be possible—modeling by system designers will be required. Baseline is to have both DM’s conjugate to the telescope pupil • One DM mounted to tip/tilt stage • This may be difficult mechanically (frequency response of t/t stage, t/t stroke (2.5 arcsecs?), DM reliability) • f/16 in, f/64 out • Exit pupil is transmissive with 15 mm diameter in converging f/64 space • Untilted image/pupil planes

5. AO relay requirements, continued • Common path wfe: < 5-10 nm for spatial frequencies 4-32 /D; ~20nm rms outside this range • Non-common path wfe: < 5 nm for spatial frequencies 4-32 /D; ~20nm rms outside this range

6. AO relay design • First-order optics requires at least 3 powered relay mirrors; 5 required in order to have collimated light at each DM • Current design has 5 relay mirrors (3 OAP’s, 1 near-sphere, 1 hyperboloid)– designs in progress to use fewer and/or simpler optics • Design Strehl ratio >0.998 at 1.0  (< 7 nm rms, mostly astigmatism) • Untilted image/pupil planes • Dimensions: ~ 1.20 m x 0.80 m

7. AO relay layout (DM folds, P&C folds removed)

10. AO relay issues • Optics manufacturability • Alignment/operational procedures

11. Wavefront sensor • Requirements • f/64 input • 4 x 4 pixels per subaperture • 1.5 arcsec / pixel plate scale • 256 x 256 CCD with 24  pixels (assumed; does not yet exist) • Design characteristics • 5.85 mm collimated beam into lenslet array • Lenslet array: 48  lenslets, f/12.6 – consistent with manufacturing capabilities of Vitrum • 2:1 magnification relay between lenslets and CCD

12. Coronagraph requirements • Input: f/64 with 15mm diameter transmissive pupil in converging space • Output: f/258, don’t care about pupil position • Occulting spot is reflective • 2nd pupil is required, 15 mm diameter, cold, in collimated space • Beamsplitter before 2nd pupil, in collimated space: reflected path to science, transmitted path to calibration unit • ~20 nm rms wfe, mid-spatial frequencies • Strehl ratio  0.97

13. Coronagraph design • 3 spherical mirrors (collimator + “telephoto”) • 2nd pupil mask is currently reflective • Output is not telecentric • Not necessary, according to science instruments • Telecentricity would have significantly increased the length or required another optic • Design Strehl ratio >0.998 at 1.0  (< 2 nm rms, mostly coma/astigmatism) • Untilted image plane • Dimensions: ~1.2 m x 0.65 m

14. Layout of coronagraph

15. Other solutions considered • Designs using one or two relay optics are possible, but— • Are long (3.75m +) and/or • Do not have collimated space for beamsplitter/cold stop • Reducing pupil mask size would reduce proportionally the distances following the occulting mask

16. Coronagraph issues • Reflective/transmissive 2nd pupil mask? • Packaging • Cold stop/baffling • Thermal issues

17. What’s left to be done • Tolerancing • Discussions with optics shops regarding manufacturability of optics with small mid-spatial frequency errors • Packaging & optomechanics • Detailed alignment plan—goes hand in hand with opto-mechanical design